Low Temperature Atomic Layer Deposition of Tin Oxide

Abstract

Atomic layer deposition (ALD) of tin oxide (SnO x ) films was achieved using a newly synthesized tin precursor and hydrogen peroxide. We obtained highly pure, conductive SnO x films at temperatures as low as 50 °C, which was possible because of high chemical reactivity between the new Sn precursor and hydrogen peroxide. The growth per cycle is around 0.18 nm/cycle in the ALD window up to 150 °C, and decreased at higher temperatures. Self-limited growth was demonstrated for both the Sn and O precursors. Thickness is linear in the number of cycles, with an induction period of not more than a few cycles. Rutherford backscattering spectroscopy (RBS) and X-ray photoelectron spectroscopy (XPS) measurements showed that the composition ratio of O/Sn is ∼2 and that the films do not contain any detectable carbon or nitrogen impurities. X-ray and electron diffraction analyses identified crystallites of SnO2 with the rutile structure and average grain size 5−10 nm. The density of the films is 83% of the bulk rutile phase. The surfaces are very smooth, with roughness about 3% of the film thickness. The lowest resistivity is about 10−2 ohm·cm. The mobility is over 7 cm2/V·s, and the free electron concentration reaches nearly 1020 cm−3. The dependence of mobility on temperature suggests that grain boundary scattering is the dominant electron scattering mechanism. The optical transmission of a 100 nm film is 87.8% and its absorption is 3.3% when averaged over the wavelengths from 400 to 800 nm. Over 80% uniformity of thickness was achieved inside holes with aspect ratios up to 50:1. This successful low temperature growth of conductive nanocrystalline SnO x films by ALD allows it to be exploited in transparent electrodes for displays, organic light emitting diodes, solar cells, conductive and protective coatings on plastic, microchannel electron multiplier plates, or as a semiconductor layer in transparent transistors.

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“…In Fig. 3 in agreement with the standard diffraction spectra of rutile SnO 2 corresponding respectively to the (1 1 0), (1 0 1) and (2 1 1) plane [29,30]. They did not change much when the SnO 2 film was sintered in a temperature range from room temperature to 200 • C. However, when the temperature was raised to 450 • C, the intensity of the three SnO 2 peaks increased significantly.…”

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“…This value of ~2 is even higher than the SnO 2 film (~1.95) grown by the combination of Sn(II) precursor and H 2 O 2 . 12 When a SnO 2 film was grown at 280 o C, we noted a small increase in refractive index (~2.05), which may be due to 45 some incorporation of a SnO-rich phase (n = 2.4) 17 coming from thermally decomposed Sn atoms (Fig. S1 †).…”

“…19 Detailed operation procedure as well as the information on the Sn(II) precursor can be found in our previous report. 12 The tin precursor was held in a bubbler at 40 70 o C, which generated a vapor pressure of 0.42 Torr. Nitric oxide (Airgas) was used as an oxidant gas for the Sn(II) precursor.…”

“…At least wt.% H 2 O 2 was used, the decomposition of the oxidant led to the limited conformality of SnO 2 on holes with the same structure. 12 From the GAXRD analysis on ~60 nm-thick SnO 2 film grown at 250 o C on a glass substrate as shown in Fig. 4b, slightly crystallized rutile SnO 2 (JCPDS #41-1445) was identified as 50 evidenced by broad peaks centered at ~30 and ~54 o .…”

“…[6][7][8][9][10][11] The authors recently reported a successful growth of SnO 2 by atomic layer deposition (ALD) at 60-250 o C by using a newly 30 synthesized cyclic amide of Sn(II) (1,3-bis(1,1-dimethylethyl)-4,5-dimethyl-(4R,5R)-1,3,2-diazastannolidin-2-ylidene) and 50 wt.% hydrogen peroxide (H 2 O 2 ). 12 The growth per cycle was as high as 1.8 Å/cycle and the lowest resistivity of ~2×10 -2 ohm·cm was obtained at a growth temperature of 120 o C. In order to get 35 conformal growth of SnO 2 on structures with high aspect-ratios, the growth temperature needed to be lowered to ~60 o C to extend the lifetime of H 2 O 2 molecules diffusing inside narrow features. The highest conformality of this approach, however, was only ~80% in a hole with aspect-ratio of ~50:1 (diameter ~200 nm).…”